Dr Atkin's diet was the absolute reverse of traditional weight-loss advice.
"Give up all forms of carbohydrate, eating whatever else you want in whatever quantity you like."
I don't know if the good doctor ever said this. I'm pretty sure that the modern version is nothing like this. But that's the folklore version, and plenty of people have tried it, and it seems to work in many cases.
An Atkins dieter has to avoid anything with carbs in it. No bread, potatoes, rice, fruit, beer, pastry, etc. etc.
No pizzas. But you can eat all the steaks and eggs and sausages and cheese and fish and vegetables that you like.
And that's absolutely ridiculous, and cannot be true, according to the traditional advice. You do exactly the wrong things, eat all the high fat stuff, and you lose weight.
You probably also make yourself very ill! If you're living on bacon and eggs and steak, where are you getting vitamins from? Why don't you just die of scurvy or something?
Let's ignore the question of vitamins. Probably no-one can stick to the pure diet. They'd be desperate for vitamins and they'd crave them. They'd probably sneak the odd fruit or the odd vitamin pill. Besides, it takes ages to die of scurvy.
If we're interested in why it works, more interesting is the fact that you need carbohydrates to burn fat. Without some sort of carbohydrate supply, you can't metabolize fat at all. How can you be burning it? What is going on?
Originally, it seems that there were two explanations for the success of the Atkins diet:
(1) The diet is so restricted and tedious that it is simply impossible to eat enough calories to make up for the exercise that you do as a normal part of daily life. Hence, weight falls off.
This was the initial reaction of diet experts. It seems unlikely to me. There are lots of things to eat that don't have carbohydrates in them.
(2) Your only sources of energy are protein and fat. The complete lack of carbohydrate disrupts your metabolism so badly that you burn fat very inefficiently. So you fail to extract much energy either from the fat you eat or from your body fat. So you lose weight fast (because you are dying!). This was Atkin's explanation.
I'm not at all sure about this one either. There is this thing called 'ketosis', that you can induce if you successfully get rid of all carbohydrates, but by that time you'd have managed to make yourself very ill indeed. I can't imagine that that sort of behaviour is any more sustainable than starvation.
But as I said before, I knew a couple of people who'd tried it, and it seemed to work, without any weird side-effects. Neither of them had taken it seriously enough to induce this ketosis thing.
A much more plausible explanation was provided by people who were into diabetes research.